For convenience, a conventional image-forming technique is explained using a color proof as an example, but almost the same concepts can be applied to other fields.
There are various systems for forming color proofs, but each system has disadvantages. As one of the typical systems for forming a color proof, there is an overlay system. In this system, multicolor images are reproduced by superposing films each having images of each primary color (e.g., red, magenta and cyan) and thus it is necessary to see images through the layers of films, which reduces the image quality.
In other system, as a system of obtaining multicolor images on one sheet, there is a surprint system. According to the system, single color images each having a primary color are successively superposed on one support sheet to form multicolor images. For example, there are known Chromarine (trade name, made by E.I. Du Pont de Nemours and Company), Match Print (trade name, made by Minesota Mining and Manufacturing Co.), Color Art (trade name, made by Fuji Photo Film Co., Ltd.), etc., but in any of these systems, 4 light-sensitive films (corresponding to each color plus grey) are required and hence waste occurs.
Furthermore, as a process for forming color images on one light-sensitive sheet, a color paper system is known. In the system, a color film original is contacted with a color photographic paper followed by exposing using each color filter and the color photographic paper is subjected to wet development to provide color images. As examples of such a system, there may be mentioned Fine Checker (trade name, made by Fuji Photo Film. Co., Ltd.) and Consensus (trade name, made by Konica Corporation). In the system, since the light-sensitive region of the color photographic paper being used is in the visible region, a dark room or an exposure apparatus equipped with a light-proof compartment is required for handling the color photographic paper. Further, the developer for use in the system must be stored and managed with specific care.
These systems have some disadvantages, for example, plural sheets are required, waste materials such as transfer sheets, toners, etc., remain, handling of light-sensitive materials in a bright room is difficult, a development system using a developer is necessary, etc., and hence improvements of the system to eliminate such disadvantages has been demanded.
As a system without having such disadvantages, the following system has been proposed.
That is, a completely dry system without waste materials can be realized by using a color image forming process which comprises using a heat-developable light-sensitive recording medium wherein latent images are formed on a light-curing composition by exposure and the components relating to coloring or discoloring are diffused in the light-sensitive material in proportion to the latent images by heating to form visible color images, the portions relating to coloring or discoloring of the uncured portions are diffused to form visible images.
There are practically many kinds of light-sensitive recording media which are used for the system and the system may be a system having a feature as a recording system of black and white images but, in particular, is useful in the case of using as a recording medium of color images.
As a practical recording medium, there is known a medium disclosed in JP-A-52-89915 (the term "JP-A" as used herein means a "unexamined published Japanese patent application"). The medium is a two-component type heat coloring medium wherein the two components, for example, an electron accepting compound and an electron donating colorless dye, are disposed inside and outside of a microcapsule containing a light-curing composition. However, in the case of this medium, even if the light-curing composition in the microcapsules is sufficiently cured, color cannot be sufficiently restrained in the cured (non-image) portion, whereby the non-imaged portions are lightly colored to tend to reduce the contrast.
As a more preferred medium causing no such coloring in the non-image portions, there is known, for example, a recording medium comprising a laminate of a layer containing a photopolymerizable composition composed of a vinyl monomer and a photopolymerization initiator, an isolating layer, and a layer composed of an electron donating colorless dye as disclosed in JP-A-61-123838. In the case of the recording medium, however, coloring of the non-imaged portion may be prevented since the thermal diffusion of the acid group scarcely occurs in the non-imaged portions, that is, the portions cured by photopolymerization.
As a process for obtaining negative images by the same system, there is, for example, a process disclosed in JP-A-60-119552. In the process, a recording medium is used wherein a photopolymerizable composition composed of a monomer or a prepolymor capable of bleaching a dye and a dye capable of being bleached by the monomer or the prepolymer are disposed isolately from each other. However, the recording medium has the similar disadvantages to the medium described above.
As the most preferred medium overcoming the problems of coloring and the low image density at the non-imaged portions, there is a recording medium described in European Patent Publication No. 412,570A2 by the same applicant. In the recording medium, one of two components of the two-component type heat-sensitive coloring medium is enclosed in microcapsules and the other component is disposed outside the microcapsules as a curing compound of a light-curing composition or together with a light-curing composition.
A recording medium for negative images using the same technical concept as above is proposed in European Patent Publication No. 412,570A2 by the same applicant. In the recording medium, a photopolymerizable composition containing an electron accepting compound and a polymerizable vinyl monomer is disposed in a layer outside microcapsules and microcapsules containing an electron donating colorless dye are disposed in the layer.
For practicing color recording using such a light-sensitive recording medium as described above, fundamentally, a recording medium having plural light sensitive layers each having sensitivity to a different light wavelength and a different coloring hue may be used. As examples of a more preferred recording medium, there are the recording media described in European Patent Publication No. 412,570A2. For example, there is a multicolor recording medium comprising a support having laminated thereon plural light-sensitive layers each being sensitive to light of a different wavelength and each coloring to a different hue, wherein the layer structure from an exposure light source side to the support side of the recording medium is successively composed of a first light-sensitive layer being sensitive to light having a central wavelength .lambda.1, an interlayer absorbing light having the central wavelength .lambda.1, a second light-sensitive layer being sensitive to light having a central wavelength .lambda.2 and coloring to a color different from the color of the 1 st light-sensitive layer, an interlayer absorbing the light having the central wavelength .lambda.(i-1), and an (i)th light-sensitive layer being sensitive to light having a central wavelength .lambda.1 and coloring to a color different from the colors of the 1st, 2nd,--(i-1)th light-sensitive layers, and the central wavelength being .lambda.1 .lambda.2&lt;--&lt;.lambda.i, wherein, i is an integer of at least 2.
Since the light-sensitive recording medium is of a monosheet type, if the central wavelength .lambda.i is set to a visible region, the medium becomes a recording medium absorbing visible light and hence the recording medium is colored overall. Accordingly, it is preferred that the central wavelength is in an ultraviolet region.
That is, in the case of a color light-sensitive recording medium having a cyan coloring layer, a magenta coloring layer, and a yellow coloring layer, it is preferred that they have three kinds of central wavelengths in an ultraviolet region. Thus, in the case of exposing such a light-sensitive material, it is preferred to use an exposure apparatus having a wavelength region as broad as possible in the ultraviolet region.
That is, in the broad wavelength region, it is preferred that the recording medium has three kinds of central wavelengths in the region of 330 nm as the lower limit, which is the short wavelength limit of light transmitting through a pressing glass of an exposure apparatus or a lithographic original, and 430 nm as the upper limit, which is allowable for coloring of the recording medium that is, over a region of about 100 nm.
However, the pressing glass of a commercially available exposure apparatus for printing is a glass which is so-called blue plate glass and hence the glass does not sufficiently transmit light having a wavelength of lower than 340 nm due to the light absorption by impurities contained in the glass. Accordingly, it is necessary to set independent three light-sensitive regions in the narrow region of substantially from 340 nm to 400 nm (even if slight coloring may be allowed, the region of from 340 nm to 420 nm), but by the common sense in the field of the art, it is very difficult to substantially divide this narrow region into the three regions.
In particular, it is difficult to obtain a monochromatic light source exposing each coloring layer only.
The narrowness of the region causes a problem that in a dichromatic light recording medium or a monochromatic light recording medium, the light sensitivity is lowered. For example, if one attempts to divide the above region of about 100 nm in the single light source into three regions using filters, an undesirable phenomenon known as cross-talk occurs. That is, for example, in the case of image-exposing the region of .lambda.2, the component light of .lambda.1 and/or .lambda.3 of light from the .lambda.2 light source passing through the filters exposes other regions or layers than the light sensitive layer for .lambda.2. Thus, if the light-sensitive recording medium thus exposed is developed, color images are formed having a color different from the color expected from the original.
Furthermore, after exposing these light sensitive recording media, heat development is necessary for stabilizing the images formed and thus exposing and heat developing apparatus capable of easily performing the heat development step have been desired.